Scientists see how and where disruptive structures form and cause voltage fading

Results: Starting as a few atoms long, thorns
forming on the electrode's surface in a specialized lithium battery cause the
battery to gradually fade, according to scientists at Pacific Northwest
National Laboratory (PNNL) and Argonne National Laboratory. Working with powerful imaging technologies in DOE's Environmental Molecular Sciences Laboratory (EMSL), the team determined that a kind of thorn with the crystallographic spinel structure grows out of
the electrode material and eventually leads to the complete conversion of the
whole electrode material into the spinel structure. Furthermore, growth of this
spinel structure liberates lithium oxide molecules, causing cracking and
pitting. The damaged electrode thereby fades, releasing less energy with each
charge/discharge cycle.

"The
changes to the structure are pretty subtle after each cyclic charge/discharge
of the battery," said Dr. Chongmin Wang, a PNNL researcher who led the
study. "Atomic-level imaging provides the opportunity to get a fundamental picture of how does this type of
subtle change evolves."

Why It Matters: Increasing our nation's independence
from fossil fuels for our transportation fleet requires energy storage. A lithium-rich layered composite could
increase batteries' energy density by more than 50 percent. However, the
battery fades. With repeated use, the voltage and amount of energy that can be
reversibly stored and released gradually declines. The cause is a change or
transformation in the composite, but how and where the transformations or phase
transitions occur was under debate. By taking and analyzing atomic-resolution
images of the battery's electrode before and after use, the team answered the
questions.

"These
findings and the follow-on studies are critical for applications, including
energy storage and electric vehicles," said Dr. Jun Liu, a key player in
the Joint Center for Energy Storage Research and a PNNL materials scientist on
the study.

Methods: The team began with layered lithium
battery electrodes, where the layers are just a single atom thick. The material
was synthesized at Argonne, where it was invented several years ago. The research team used a new energy
dispersive spectrometer (EDS) and a powerful scanning transmission electron
microscope to obtain detailed chemical composition and atomic structure information
on the electrode materials. The FEI Company, in Hillsboro, Oregon, supplied the
EDS. The company was looking for outstanding examples to showcase the power of
their instruments. Using the EDS spectrometer, the team identified chemical
inhomogeneity and its correlation with the phase changes that occurred in the
material.

The team used
a new electron microscope located in EMSL to obtain atomic-resolution images.
"The high-resolution work is cutting-edge imaging research," said Dr.
Nigel Browning, Chief Scientist in Microscopy for the Chemical Imaging
Initiative at PNNL and researcher on this study. "It is a fantastic
application of atomic-resolution microscopy techniques, and it confirms that
spinel formation can account for the origin of voltage fading by determining
the exact location of the spinel, and how the whole structure fragments as the
spinels are formed."

What's Next? Many of the people on this project are
working to find new ways to synthesize and stabilize the materials in the
layered lithium battery. "We are working as a team to take it to the next
level - in situ images. We want to
see the changes at the atomic level as they occur," said Wang.